Pre-distortion circuit, apparatus, method and computer program for pre-distorting, transmitter, radio transceiver, mobile transceiver, base station transceiver, communication device, storage

ABSTRACT

Embodiments provide a pre-distortion circuit and apparatus, a method and computer program for pre-distorting, a transmitter, a radio transceiver, a communication device, a mobile transceiver, a base station transceiver and a storage. The pre-distortion circuit ( 10 ) is configured for a digital quadrature signal. The pre-distortion circuit ( 10 ) comprises a first input ( 12 ) for an inphase component of the quadrature signal and a second input ( 14 ) for a quadrature component of the quadrature signal. The pre-distortion circuit  10  comprises a signal processing circuit ( 16 ) configured to determine whether polarities of the inphase component and quadrature component are equal, and to determine pre-distortion coefficients based on the amplitude of the inphase component, the amplitude of the quadrature component, and based on whether the polarities are equal.

FIELD

Examples relate to pre-distortion of components of a quadrature signaland in particular, but not exclusively, to pre-distortion of thecomponents of a quadrature signal before radiofrequencydigital-to-analog-conversion in a transmitter.

BACKGROUND

Digital communication systems are well established. Data applicationshave been available for many years and with the developing standard forcellular systems digital communication becomes more important. At somepoint in a transmitter digital signals are converted into analog signalsbefore the analog signals are transmitted at Radio Frequency (RF).Digital-to-analog conversion is a component in the communication chain.It is used to translate or convert signals from a digital domain to ananalog domain before the signals can be transmitted over an antenna.Recent developments in the field of digital-to-analog conversion havebrought Radio-Frequency-Digital-to-Analog-Converters (RFDAC), in someliterature also known as Direct-Digital-DAC. An RFDAC combinesfunctionalities of a digital-to-analog converter and a mixer into asingle component, and since it allows highly integrated implementationwith reduced size and utilization of advantages of digital circuits, itis already used by some devices.

There are many implementation possibilities of RFDACs, such ascurrent-DAC (or I-DAC), or resistance-DAC (or R-DAC), and capacitive-DAC(or C-DAC). Because of its advantages compared to others, C-DAC may bepreferable in some products.

BRIEF DESCRIPTION OF THE FIGURES

Some examples of circuits, apparatuses, transmitters, methods, computerprograms, etc. will be described in the following by way of exampleonly, and with reference to the accompanying figures, in which

FIG. 1 illustrates an example of a pre-distortion circuit and an exampleof a pre-distortion apparatus;

FIG. 2 illustrates a phase to amplitude relation of a C-DAC in anexample;

FIG. 3 shows a local oscillator signal timing for a quadrature signal inan example;

FIG. 4 illustrates a phase offset calculation for IQ C-DAC in anexample;

FIG. 5 shows IQ and polar representations of distorted signals in anexample;

FIG. 6 shows a circuit diagram of an implementation of an example;

FIG. 7 illustrates examples of a mobile communication system, a mobiletransceiver, and a base station transceiver; and

FIG. 8 shows a block diagram of an example of a method forpre-distorting.

DETAILED DESCRIPTION

Various examples will now be described more fully with reference to theaccompanying drawings in which some examples are illustrated. In thefigures, the thicknesses of lines, layers and/or regions may beexaggerated for clarity.

Accordingly, while further examples are capable of various modificationsand alternative forms, some examples thereof are shown by way of examplein the figures and will herein be described in detail. It should beunderstood, however, that there is no intent to limit examples to theparticular forms disclosed, but on the contrary, examples are to coverall modifications, equivalents, and alternatives falling within thescope of the disclosure. Like numbers refer to like or similar elementsthroughout the description of the figures.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” etc.).

The terminology used herein is for the purpose of describing particularexamples only and is not intended to be limiting of further examples. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes” and/or “including,” when used herein, specifythe presence of stated features, integers, steps, operations, elementsand/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which examples belong. It will befurther understood that terms, e.g., those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art, unlessexpressly defined otherwise herein.

In the following some examples will be described. FIG. 1 illustrates anexample of a predistortion circuit 10, an example of a pre-distortionapparatus 10, respectively. FIG. 1 shows a pre-distortion circuit 10 fora digital quadrature signal. The pre-distortion circuit 10 comprises afirst input 12 for an inphase component of the quadrature signal and asecond input 14 for a quadrature component of the quadrature signal. Thefirst and second inputs 12, 14 are coupled to a signal processingcircuit 16. The signal processing circuit 16 is configured to determinethe signs of the inphase component and of the quadrature component. Thesignal processing circuit 16 is further configured to determinepre-distortion coefficients based on the amplitude of the inphasecomponent, the amplitude of the quadrature component, and based on thesigns of the inphase component and the quadrature component. Thepredistortion coefficients may then be used to modify the inphase andquadrature components to obtain pre-distorted versions of the inphasesignal and the quadrature signal.

FIG. 1 also illustrates an example of a pre-distortion apparatus 10 fora digital quadrature signal. The pre-distortion apparatus 10 comprises afirst input 12 for an inphase component of the quadrature signal and asecond input 14 for a quadrature component of the quadrature signal. Theapparatus 10 comprises signal processing means 16 configured fordetermining signs of the inphase component and of the quadraturecomponent. The processing means 16 is further configured for determiningpre-distortion coefficients based on the amplitude of the inphasecomponent, the amplitude of the quadrature component, and based on thesigns of the inphase component and the quadrature component.

In the following further examples will be described and implementationdetails for components of the pre-distortion circuit 10 orpre-distortion apparatus 10. Such details of the determination circuit10 may be likewise applied to the determination apparatus 10, even ifexplicit repetition is omitted, “module” features may correspond to therespective “means” features.

In examples the first and second inputs 12 and 14 may be implemented ascontacts, pins or input registers that allow providing the correspondingsignals to the processing circuit 16.

The inphase and quadrature component may be digital signals and thesignal processing circuit 16 may be correspondingly configured. Thesignal processing circuit 16 may comprise digital components. Inexamples signal processing circuit 16 may be implemented using one ormore processing units, one or more processing devices, any means forprocessing, such as a processor, a computer or a programmable hardwarecomponent being operable with accordingly adapted software. In otherwords, the described function of signal processing circuit 16 may aswell partly or completely be implemented in software, which is thenexecuted on one or more programmable hardware components. Such hardwarecomponents may comprise a general purpose processor, a Digital SignalProcessor (DSP), a microcontroller, etc. In examples the signalprocessing circuit 16 may comprise logical components, such as logicalgates, switches, multiplexers, registers, arithmetic logic units, etc.

In some examples the signs of the inphase component and the quadraturecomponent are explicitly determined, such that four combinations of thesigns are possible. The signal processing circuit 16 may be configuredto determine whether polarities of the inphase component (I) andquadrature component (Q) are equal to 1 or to −1. This way, examples mayprovide the possibility to distinguish four different cases,

1. sign (I)=1, sign(Q)=1;2. sign (I)=−1, sign(Q)=1;3. sign (I)=1, sign(Q)=−1; and4. sign (I)=−1, sign(Q)=−1.

For each of these cases the amplitudes of the respective components maybe used differently in order to determine the pre-distortioncoefficients. Some examples may use one or more functional relationsbetween the signs, the amplitudes of the components and thepredistortion coefficients. Other examples may use Look-Up Tables (LUT)with corresponding mappings between pre-distortion coefficients andamplitudes as will be detailed subsequently. For example, four LUTs maybe distinguished for the four cases as set out above. Examples may covercases where this pre-distortion can be implemented according to thepossibilities of sign(I) and sign(Q). In case of a symmetric behavior(same behavior for cases 1 and 4, and for cases 2 and 3), equality ofthe signs or polarities of the signal components may be determined. In ageneral example the signs of I and Q are evaluated, and based on theevaluation the coefficients are determined based on the signs of I andQ.

As further indicated in FIG. 1 in some examples the signal processingcircuit 16 may be configured to determine a pre-distorted version of theinphase component and a pre-distorted version of the quadraturecomponent. For example, the pre-distorted version of the inphasecomponent may be an addition of the inphase component and the quadraturecomponent weighted with the pre-distortion coefficient. Thepre-distorted version of the quadrature component may be an addition ofthe quadrature component and the inphase component weighted with thepre-distortion coefficient. Examples also provide a transmitter 100comprising the pre-distortion circuit 10, pre-distortion apparatus 10,respectively.

In an example an RFDAC, which is comprised in a transmitter, isconfigured for converting a base band signal to an analog radiofrequency signal based an output signal (e.g. predistorted version ofthe inphase and quadrature components) of the pre-distortion circuit 10.For example, a C-DAC may be used. C-DAC may have intrinsic AmplitudeModulation (AM) and Phase Modulation (PM) effects, depending on a numberof currently turned on cells (AM) and a cell switching delay(corresponding to phase offset or PM) changes. Such an effect may bedecreased for a polar architecture C-DAC using standard AMPMpredistortion. In case a quadrature signal is represented usingquadrature (Q) and inphase (I) components standard AMPM pre-distortionmay be non-optimal due to IQ cross-effects. Examples may use a lowcomplexity AMPM pre-distortion for IQ C-DAC. Using a multidimensionalDigital Pre-Distortion (DPD), e.g. 2-dimensional DPD, may result in asignal processing task.

The phase error may depend on a number of currently turned on cells,which corresponds to the input amplitude of the C-DAC (the higher theinput amplitude of the C-DAC the more cells are turned on or activated).In general an RFDAC may comprise a serial or a parallel structure ofsimilar sub-structures, which are also referred to as cells. Such asub-structure may comprise one or more transistors, capacitors,resistors, inductors, etc. For example, there is a total number of Ncells, and each cell can be activated or de-activated, e.g. using atransistor. For example, cells are activated until a certain inputamplitude is matched (exceeded), capacitors or inductors (low passfilters in general) may then be used to eliminate sharp edges and toobtain an analog output signal. Such components may as well be comprisedin a matching network of the RFDAC. The more sub-structures areactivated the higher an influence of gate capacities, for example.

On the one hand, the complexity in IQ is that the number of “currentlyturned on cells” seen by I or Q during switching varies depending on I&Qsigns (whether their polarities are equal or different), and can beequal to III, IQI or III+IQI. On the other hand, a relationship of thenumber of “currently turned on cells” or amplitude and the PM is quitestable and well characterized. The basic principle of the pre-distortionin some examples is to define or determine which condition applies to Iand Q for the given signs, to define a low complex mechanism for AMPMpre-distortion of an IQ modulator, and to pre-distort I and Q separatelyusing at least one 1-dimensional DPD Look-up table (LUT). In general,LUTs may be distinguished according to the four combinations (casespointed out above) of the signs of I and Q. For example, a linearapproximation of AMPM distortion can be used. As further shown in FIG. 1the pre-distortion circuit 10 may further comprise a generation module18 configured to generate the pre-distortion coefficients. Suchgeneration may be based on a functional relation between thepre-distortion coefficients and the amplitudes of I and Q, the signs ofI and Q, respectively. Some examples may distinguish differentfunctional relations for different combinations of the signs of I and Q.In some examples the pre-distortion coefficients may be calculatedinstead of using a LUT. In a general example, the generation module 18may use the signs and amplitudes of I and Q as input parameters andoutputs the pre-distortion coefficients. Using one or more LUTs is justan example for an efficient hardware implementation of the generationmodule 18. The generation module 18 may be implemented as a storagemodule configured to store at least one one-dimensional look-up table.

The storage module is an optional component. In examples such a memoryor storage may be a flash memory, a volatile- or non-volatile memory,Random Access Memory (RAM), Dynamic RAM (DRAM), Static RAM (SRAM), orany kind of memory.

Comparing to 2D look-up tables, examples may provide lower complexity,smaller area and lower power consumption. The reason can be seen in thefollowing example. If a suppression of a third order nonlinearity isneeded, the following polynomial may be generated:

I_(pred)=α₁I+α₂Q+α₃IQ+α₄I₂+α₅Q²+α₆I²Q+α₇IQ²+α₈I³+α₉Q³,Q_(pred)=β₁I+β₂Q+β₃IQ+β₄I²+β₅Q²+β₆I²Q+β₇IQ²+β₈I³+β₉Q³.which means approximately 40 multipliers, 18 adders and 18 coefficients.All these coefficients need to be calculated from the 2D LUT. For thehigher order nonlinearity the complexity increases almost exponentially.Such solutions might not be likely to be implemented since they mayreach limitations in complexity, area and power consumption.

Examples may offer lower complexity and lower effort for implementationsince the following terms may be generated:

I_(pred)=I+α_(Q)QQ_(pred)=−α_(I)I+Q

Thus, some examples may need 3 adders and two multiplications. Twocoefficients are calculated using a 1D LUT in some examples. The atleast one look-up table may comprise a mapping between amplitude valuesand pre-distortion coefficients or the at least one lookup table maycomprise a mapping between a number of activated cells in aradio-frequency-digital-to-analog converter and pre-distortioncoefficients. The radio-frequency-digital-to-analog converter isconfigured to convert the inphase component and/or the quadraturecomponent.

Examples may be further based on the finding that the phase offset isrelated to the ratio of the turned on and off cells of a C-DAC. FIG. 2illustrates a phase to amplitude relation of a C-DAC in an example. FIG.2 shows a typical AMPM measurement for a polar modulator. The abscissarepresents the normalized input amplitude, which is directly related tothe number of turned on cells and the ordinate represents the phaseoffset. Cells of the C-DAC are switched on and off according to: 1) theinput amplitude and 2) LO signal. Input amplitude determines how manycells are needed to represent given amplitude (or I/Q) and the LOsignals are used to modulate a given amplitude. This means when the LOsignal equals 1 the cells are turned on, and when the LO signal equalszero cells are switched off.

FIG. 3 shows a local oscillator signal timing for a quadrature signal inan example. FIG. 3 shows time changes of the LO signals in I (broken ordashed line) and (solid line) Q and corresponding amplitude changes.FIG. 3 shows the inphase and quadrature components of the LO signals.These are the LO signals, the above described inphase and quadraturecomponents get mixed with, for example, in an RFDAC. The timing can besub-divided in four equidistant periods each having a quarter durationof the LO period. In the first period LO Q=1 and LO I=0, in the secondperiod LO Q=1 and LO I=1, in the third period LO Q=0 and LO I=1, and thefourth period LO Q=0 and LO 1=0. FIG. 3 illustrates the situation whenthe polarities of I (inphase component) and Q (quadrature component) areequal, hence LO Q has the first rising edge. If they differ, thesituation is mirrored and LO I has the first rising edge. This means,when I and Q have the same sign, the LO Q signal changes to 1 during thefirst time slot, while the LO I is 0. Thus the Q signal “sees” the wholearray of cells turned off, and the generated phase offset corresponds tothe amplitude in Q. After a quarter of the LO period the LO I changesto 1. I signal now “sees” Q cells that are turned on and N-Q cells thatare off (where N is the total number of cells of the C-DAC). Thus thegenerated phase offset of the I signal corresponds to the (III+IQI)amplitude. This situation is further illustrated FIG. 4. FIG. 4illustrates a phase offset calculation for IQ C-DAC in an example. Theillustration of FIG. 4 is similar to the illustration of FIG. 2. Duringthe second period AMPM is determined based on III+IQI.

This applies when the signs of the I and Q signals are equal. Otherwisethe LO I signal starts a quarter of the LO period before LO Q. For thisreason examples may use for both I and Q a 1D LUT that describes AMPMbehavior of the C-DAC, for the case of equal signs input III+IQI andIQI, and if the signs are not equal examples may use III and III+IQI.Hence, in some examples the signal processing circuit 16 may beconfigured to determine the predistortion coefficients from the at leastone look-up table based on a magnitude of the quadrature component andbased on a sum of magnitudes of the quadrature component and the inphasecomponent. The signal processing circuit 16 may be configured todetermine the pre-distortion coefficients from the at least one look-uptable based on a magnitude of the inphase component and based on a sumof magnitudes of the quadrature component and the inphase component.

As the phase offset is defined therewith the next step is to define thepre-distortion procedure of the IQ modulator. Both amplitudes generatephase offsets, see FIG. 4. FIG. 5 shows IQ and polar representations ofdistorted signals in an example. FIG. 5 shows an AMPM and AMAMrepresentation for an IQ transmitter on the left and for a polartransmitter on the right. Both representations are vectorrepresentations in the complex plane. As shown on the left, the inputsignal Q experiences AMAM, AMPM and gets distorted to the output signalQ. Similar distortion applies to the input signal I and the distortedoutput signal I Likewise the distortion modified the polar input signal(shown on the right) to a distorted output signal.

As represented in FIG. 5, there is a connection between the polar andthe IQ transmitter, such that AMAM and AMPM for IQ are as if there weretwo polar vectors that work together and are orthogonal to each other.In the IQ modulation all signals that need to be transmitted arerepresented as linear combination of the two basis vectors, which areorthogonal to each other. AMPM offset can be now defined as change ofthe basis, where base vectors are multiplied with a rotational matrix.Both I and Q experience phase offsets that are not necessarily equal. Asa result a new basis with the base vectors I′ and Q′ can be obtainedwhose difference is not 90° anymore, i.e. I and Q are no longerorthogonal meaning that IQ cross talk is created. Expressed in equationsthis yields:

${\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix} = {\begin{bmatrix}{\cos( \propto_{I} )} & {\sin( \propto_{Q} )} \\{- {\sin( \propto_{I} )}} & {\cos( \propto_{Q} )}\end{bmatrix}\begin{bmatrix}I \\Q\end{bmatrix}}},{{{or}\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix}} = {H_{non}\begin{bmatrix}I \\Q\end{bmatrix}}},$

where ∝_(I) and ∝_(Q) are the AMPM phase offsets in I and Qrespectively, and H_(non) is the rotational matrix used for the basistransformation. This represents a finding of examples, namely thepre-distortion for IQ is a reciprocal rotational matrix:

$H_{pred} = {H_{non}^{- 1} = {\begin{bmatrix}{\cos( \propto_{I} )} & {\sin( \propto_{Q} )} \\{- {\sin( \propto_{I} )}} & {\cos( \propto_{Q} )}\end{bmatrix}^{- 1}.}}$

This relation can be simplified. First, if small angles are assumed,which is generally true, then the cosine is approximately equal to 1 andthe sine is approximately equal to the given angle. Second, anothersimplification is related to the fact that for the basis transformationthe rotational matrix has been used. Thus to avoid matrix inversionnegative angles could be used. So a further approximation can be definedas

$H_{pred} = {\begin{bmatrix}1 & \propto_{Q} \\{- \propto_{I}} & 1\end{bmatrix}.}$

In some examples, the at least one look-up table may comprise anapproximation of a rotation of a polar representation of the quadraturesignal to the inphase and quadrature components. The at least onelook-up table may comprises an approximation assuming non-orthogonalrotations of the polar representation to obtain the inphase andquadrature components. The approximation may be linear.

FIG. 6 shows a circuit diagram of an implementation of an example of adetection circuit 10. FIG. 6 shows on the left hand side an input 12 forthe inphase component Sig I and an input 14 for the quadrature componentSig Q. In the following details of the signal processing circuit 16 inthe example will be described. The signal processing circuit 16comprises two blocks 16 a and 16 b, which determine signs of the inphasecomponent Sign(Sig I) and the quadrature component Sign(Sig Q). Acomparator 16 c then determines whether the polarities of the inphasecomponent and the quadrature component are equal or not. Based on anoutput of the comparator 16 c two multiplexers 16 d and 16 e can becontrolled, which each select between two different inputs depending onthe polarity relation. Two blocks 16 f and 16 g, which are in parallelto the blocks 16 a and 16 b, determine the magnitudes of the Abs(SigI)=III of the inphase component and Abs(Sig Q)=IQI of the quadraturecomponent and the summation element 16 h provides the sum of themagnitudes III+IQI. The multiplexers 16 d and 16 e can then selectbetween III+IQI and III or QI, respectively, based on the control signalfrom the comparator 16 c, i.e. based on whether the polarities areequal, or more generally whether the sign(I)=1 and sign(Q)=1, orsign(I)=−1 and sign(Q)=−1. In case the polarities are equal, IQI isselected by multiplexer 16 e and III+IQI is selected by multiplexer 16d. In case the polarities are not equal III is selected by multiplexer16 d and III+IQI is selected by multiplexer 16 j. The selected valuesare then input into two 1D LUTs 16 i and 16 j, which output therespective pre-distortion coefficients ∝_(I) and ∝_(Q).

In the example shown in FIG. 6 two 1D LUTs 16 i and 16 j are shown. Inanother example a single 1D LUT may be used to determine the respectivecoefficients based on the input amplitude, cf. FIGS. 3 and 5. In someexamples the same 1D LUT may be used to determine the pre-distortioncoefficients from. In the example shown in FIG. 6 the signal processingcircuit 16 is configured to, in case the polarities of the inphasecomponent and the quadrature component are equal, determine thepre-distortion coefficients from the at least one look-up table 16 i, 16j based on a magnitude of the quadrature component and based on a sum ofmagnitudes of the quadrature component and the inphase component. Incase the polarities of the inphase component and the quadraturecomponent are not equal the signal processing circuit 16 is configuredto determine the pre-distortion coefficients from the at least onelook-up table 16 i, 16 j based on a magnitude of the inphase componentand based on a sum of magnitudes of the quadrature component and theinphase component.

Furthermore, in the example in FIG. 6 the signal processing circuit 16is configured to determine a pre-distorted version of the inphasecomponent and a pre-distorted version of the quadrature component. Thisis implemented using two multipliers 16 k and 16 l. Multiplier 16 kdetermines ∝_(I) I based on the inphase component I and ∝_(I).Multiplier 16 l determines ∝_(Q) Q based on the quadrature component Qand ∝_(Q). Two adders 16 m and 16 n then determine pre-distortedversions of the inphase component and the quadrature component, namelyI+∝_(Q) Q as pre-distorted version of the inphase signal and Q−∝_(I) Ias pre-distorted quadrature component. The pre-distorted version of theinphase component is an addition of the inphase component and thequadrature component weighted with the predistortion coefficient. Thepre-distorted version of the quadrature component is an addition of thequadrature component and the inphase component weighted with thepre-distortion coefficient.

Summarizing the example shown in FIG. 6 first amplitudes of the I and Qsignals are added 16 f, 16 g, 16 h and the signs are compared 16 a, 16b, 16 c. If the signs are equal then as an input of the 1D LUT 16 i, 16j yields III+IQI and IQI. If the signs are not equal the inputs are IIIand III+IQI. The 1D LUT is designed according to the measured AMPMbehavior and it is approximated as linear function. Calculatedcoefficients are then multiplied with signals in I and Q and added tothe signals as represented in FIG. 7. In an example of a transmitter 100the pre-distorted versions of the inphase and quadrature components arethen provided to an RFDAC for analog-to-digital conversion andconversion to the transmission band. In another example four LUTs may beused depending on the four sign combinations.

Another example of a transmitter 100 is illustrated in FIG. 7. Thetransmitter comprises an example of the pre-distortion circuit as shownin FIG. 6 and an RFDAC 20. The output of the RFDAC is provided to aPower Amplifier 22 before the signal is being transmitted. It is to benoted that the transmitter may comprise further components, such as oneor more filters, diplexers, duplexers, antennas, etc. Such a transmitter100 may be comprised in a radio transceiver, such as a mobiletransceiver 200 or base station transceiver 300. Examples also provide acommunication device comprising the radio transceiver. FIG. 7illustrates examples of a mobile communication system 400, a mobiletransceiver 200, and a base station transceiver 300. Examples alsoprovide a mobile communication system 400 comprising a mobiletransceiver 200 and a base station transceiver 300 as also illustratedby FIG. 7.

The mobile communication system 400 may correspond, for example, to oneof the Third Generation Partnership Project (3GPP)-standardized mobilecommunication networks, where the term mobile communication system isused synonymously to mobile communication network. The mobile orwireless communication system may correspond to a mobile communicationsystem of the 5th Generation (5G) and may use mm-Wave technology. Themobile communication system may correspond to or comprise, for example,a Long-Term Evolution (LTE), an LTE-Advanced (LTE-A), High Speed PacketAccess (HSPA), a Universal Mobile Telecommunication System (UMTS) or aUMTS Terrestrial Radio Access Network (UTRAN), an evolved-UTRAN(e-UTRAN), a Global System for Mobile communication (GSM) or EnhancedData rates for GSM Evolution (EDGE) network, a GSM/EDGE Radio AccessNetwork (GERAN), or mobile communication networks with differentstandards, for example, a Worldwide Interoperability for MicrowaveAccess (WIMAX) network IEEE 802.16 or Wireless Local Area Network (WLAN)IEEE 802.11, generally an Orthogonal Frequency Division Multiple Access(OFDMA) network, a Time Division Multiple Access (TDMA) network, a CodeDivision Multiple Access (CDMA) network, a Wideband-CDMA (WCDMA)network, a Frequency Division Multiple Access (FDMA) network, a SpatialDivision Multiple Access (SDMA) network, etc.

A base station or base station transceiver 300 can be operable tocommunicate with one or more active mobile transceivers 200 and a basestation transceiver 300 can be located in or adjacent to a coverage areaof another base station transceiver, e.g. a macro cell base stationtransceiver or small cell base station transceiver. Hence, examples mayprovide a mobile communication system 400 comprising one or more mobiletransceivers 200 and one or more base station transceivers 300, whereinthe base station transceivers 300 may establish macro cells or smallcells, as e.g. pico-, metro-, or femto cells. A mobile transceiver 200may correspond to a smartphone, a cell phone, user equipment, a laptop,a notebook, a personal computer, a Personal Digital Assistant (PDA), aUniversal Serial Bus (USB)-stick, a car, etc. A mobile transceiver 200may also be referred to as User Equipment (UE) or mobile in line withthe 3GPP terminology.

A base station transceiver 300 can be located in the fixed or stationarypart of the network or system. A base station transceiver 300 maycorrespond to a remote radio head, a transmission point, an accesspoint, a macro cell, a small cell, a micro cell, a femto cell, a metrocell etc. A base station transceiver 300 can be a wireless interface ofa wired network, which enables transmission of radio signals to a UE ormobile transceiver 200. Such a radio signal may comply with radiosignals as, for example, standardized by 3GPP or, generally, in linewith one or more of the above listed systems. Thus, a base stationtransceiver 300 may correspond to a NodeB, an eNodeB, a Base TransceiverStation (BTS), an access point, a remote radio head, a transmissionpoint etc., which may be further subdivided in a remote unit and acentral unit.

FIG. 8 shows a block diagram of an example of a method forpre-distorting. The predistortion method is configured for a digitalquadrature signal. The pre-distortion method comprises inputting 32 aninphase component of the quadrature signal and inputting 34 a quadraturecomponent of the quadrature signal. The method further comprisesdetermining 36 the signs of the inphase component and of the quadraturecomponent. The method comprises determining 38 pre-distortioncoefficients based on the amplitude of the inphase component, theamplitude of the quadrature component, and based on the signs of theinphase component and the quadrature component.

Another example is a computer program having a program code forperforming at least one of the above methods, when the computer programis executed on a computer, a processor, or a programmable hardwarecomponent. Yet another example is a computer readable storage mediumstoring instructions which, when executed by a computer, processor, orprogrammable hardware component, cause the computer to implement one ofthe methods described herein.

The examples as described herein may be summarized as follows:

A first example is a pre-distortion circuit 10 for a digital quadraturesignal. The predistortion circuit 10 comprises a first input 12 for aninphase component of the quadrature signal, and a second input 14 for aquadrature component of the quadrature signal. The predistortion circuit10 further comprises a signal processing circuit 16 configured todetermine the signs of the inphase component and of the quadraturecomponent, and to determine predistortion coefficients based on theamplitude of the inphase component, the amplitude of the quadraturecomponent, and based on the signs of the inphase component and thequadrature component.

In example 2 the pre-distortion circuit 10 of example 1 comprises ageneration module 18 configured to generate the pre-distortioncoefficients.

Example 3 is the pre-distortion circuit 10 of example 2, wherein thegeneration module 18 is a storage module configured to store at leastone one-dimensional look-up table, and wherein the at least one look-uptable comprises a mapping between amplitude values and predistortioncoefficients or wherein the at least one look-up table comprises amapping between a number of activated cells in aradio-frequency-digital-to-analog converter and predistortioncoefficients, the radio-frequency-digital-to-analog converter beingconfigured to convert the inphase component or the quadrature component.

Example 4 is the pre-distortion circuit 10 of example 3, wherein thesignal processing circuit 16 is configured to determine a pre-distortioncoefficient from the at least one look-up table based on a magnitude ofthe quadrature component in case the polarities are equal and whereinthe signal processing circuit 16 is configured to determine apre-distortion coefficient from the at least one look-up table based ona magnitude of the inphase component in case the polarities aredifferent.

Example 5 is the pre-distortion circuit 10 of one of the examples 3 or4, wherein the at least one look-up table comprises an approximation ofa rotation of a polar representation of the quadrature signal to theinphase and quadrature components.

Example 6 is the pre-distortion circuit 10 of example 5, wherein the atleast one look-up table comprises an approximation assumingnon-orthogonal rotations of the polar representation to obtain theinphase and quadrature components.

Example 7 is the pre-distortion circuit 10 of one of the examples 5 or6, wherein the approximation is linear.

Example 8 is the pre-distortion circuit 10 of one of the examples 3 to7, wherein the signal processing circuit 16 is configured to determinepre-distortion coefficients from the at least one look-up table based ona magnitude of the quadrature component and based on a sum of magnitudesof the quadrature component and the inphase component.

Example 9 is the pre-distortion circuit 10 of one of the examples 3 to8, wherein the signal processing circuit 16 is configured to determinethe pre-distortion coefficients from the at least one look-up tablebased on a magnitude of the inphase component and based on a sum ofmagnitudes of the quadrature component and the inphase component.

Example 10 is the pre-distortion circuit 10 of one of the examples 3 to9, wherein the signal processing circuit 16 is configured to, in casethe polarities of the inphase component and the quadrature component areequal, determine the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the quadrature component and basedon a sum of magnitudes of the quadrature component and the inphasecomponent, and, in case the polarities of the inphase component and thequadrature component are not equal, determine the pre-distortioncoefficients from the at least one look-up table based on a magnitude ofthe inphase component and based on a sum of magnitudes of the quadraturecomponent and the inphase component.

Example 11 is the pre-distortion circuit 10 of one of the examples 1 to10, wherein the signal processing circuit 16 is configured to determinea pre-distorted version of the inphase component and a pre-distortedversion of the quadrature component.

Example 12 is the pre-distortion circuit 10 of example 11, wherein thepre-distorted version of the inphase component is an addition of theinphase component and the quadrature component weighted with thepre-distortion coefficient.

Example 13 is the pre-distortion circuit 10 of one of the examples 1 to12, wherein the predistorted version of the quadrature component is anaddition of the quadrature component and the inphase component weightedwith the pre-distortion coefficient.

Example 14 is a pre-distortion apparatus 10 for a digital quadraturesignal. The predistortion apparatus 10 comprises a first input 12 for aninphase component of the quadrature signal, and a second input 14 for aquadrature component of the quadrature signal. The pre-distortionapparatus 10 comprises signal processing means 16 configured fordetermining signs of the inphase component and of the quadraturecomponent, and for determining pre-distortion coefficients based on theamplitude of the inphase component, the amplitude of the quadraturecomponent, and based on the signs of the inphase component and thequadrature component.

Example 15 is the pre-distortion apparatus 10 of example 14, comprisinga generation means 18 configured for generating the pre-distortioncoefficients.

Example 16 is the pre-distortion apparatus 10 of example 15, wherein thegeneration means 18 comprises storing means configured for storing atleast one one-dimensional look-up table, and wherein the at least onelook-up table comprises a mapping between amplitude values andpre-distortion coefficients or wherein the at least one look-up tablecomprises a mapping between a number of activated cells in aradio-frequency-digital-to-analog converter and pre-distortioncoefficients, the radio-frequency-digital-to-analog converter beingconfigured for converting the inphase component or the quadraturecomponent.

Example 17 is the pre-distortion apparatus 10 of example 16, wherein thesignal processing means 16 is configured for determining thepre-distortion coefficients from the at least one look-up table based ona magnitude of the quadrature component in case the polarities are equaland wherein the signal processing means 16 is configured for determiningthe predistortion coefficients from the at least one look-up table basedon a magnitude of the inphase component in case the polarities aredifferent.

Example 18 is the pre-distortion apparatus 10 of one of the examples 16or 17, wherein the at least one look-up table comprises an approximationof a rotation of a polar representation of the quadrature signal to theinphase and quadrature components.

Example 19 is the pre-distortion apparatus 10 of example 18, wherein theat least one lookup table comprises an approximation assumingnon-orthogonal rotations of the polar representation to obtain theinphase and quadrature components.

Example 20 is the pre-distortion apparatus 10 of one of the examples 18or 19, wherein the approximation is linear.

Example 21 is the pre-distortion apparatus 10 of one of the examples 16to 20, wherein the signal processing means 16 is configured fordetermining the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the quadrature component and basedon a sum of magnitudes of the quadrature component and the inphasecomponent.

Example 22 is the pre-distortion apparatus 10 of one of the examples 16to 21, wherein the signal processing means 16 is configured fordetermining the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the inphase component and based ona sum of magnitudes of the quadrature component and the inphasecomponent.

Example 23 is the pre-distortion apparatus 10 of one of the examples 16to 22, wherein the signal processing means 16 is configured for, in casethe polarities of the inphase component and the quadrature component areequal, determining the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the quadrature component and basedon a sum of magnitudes of the quadrature component and the inphasecomponent, and, in case the polarities of the inphase component and thequadrature component are not equal, determining the pre-distortioncoefficients from the at least one look-up table based on a magnitude ofthe inphase component and based on a sum of magnitudes of the quadraturecomponent and the inphase component.

Example 24 is the pre-distortion apparatus 10 of one of the examples 14to 23, wherein the signal processing means 16 is configured fordetermining a pre-distorted version of the inphase component and apre-distorted version of the quadrature component.

Example 25 is the pre-distortion apparatus 10 of example 24, wherein thepre-distorted version of the inphase component is an addition of theinphase component and the quadrature component weighted with thepre-distortion coefficient.

Example 26 is the pre-distortion apparatus 10 of one of the examples 14to 25, wherein the pre-distorted version of the quadrature component isan addition of the quadrature component and the inphase componentweighted with the pre-distortion coefficient.

Example 27 is a transmitter 100 comprising the pre-distortion circuit 10of any of the examples 1 to 13 or the pre-distortion apparatus 10 of anyof the examples 14 to 26, and a radiofrequency-digital-to-analogconverter 20 configured for converting a base band signal to an analogradio frequency signal based an output signal of the pre-distortioncircuit 10.

Example 28 is the transmitter 100 of example 27, wherein theradio-frequency-digital-to-analog converter is a capacitivedigital-to-analog converter.

Example 29 is a radio transceiver comprising the transmitter 100 of oneof the examples 27 or 28.

Example 30 is a mobile transceiver 200 comprising the radio transceiverof example 29.

Example 31 is a base station transceiver 300 comprising the radiotransceiver of example 29.

Example 32 is a pre-distortion method for a digital quadrature signal.The pre-distortion method comprises inputting 32 an inphase component ofthe quadrature signal and inputting a 34 quadrature component of thequadrature signal. The method comprises determining 36 signs of theinphase component and of the quadrature component, and determining 38predistortion coefficients based on the amplitude of the inphasecomponent, the amplitude of the quadrature component, and based on thesigns of the inphase component and the quadrature component.

Example 33 is the pre-distortion method of example 32, comprisinggenerating 18 the predistortion coefficients.

Example 34 is the pre-distortion method of example 33, wherein thegenerating (18) comprises storing at least one one-dimensional look-uptable, and wherein the at least one lookup table comprises a mappingbetween amplitude values and pre-distortion coefficients or wherein a atleast one look-up table comprises a mapping between a number ofactivated cells in a radio-frequency-digital-to-analog converter andpre-distortion coefficients, the radio-frequency-digital-to-analogconverter being configured for converting the inphase component or thequadrature component.

Example 35 is the pre-distortion method of example 34, comprisingdetermining a predistortion coefficient from the at least one look-uptable based on a magnitude of the quadrature component in case thepolarities are equal, determining a pre-distortion coefficient from theat least one look-up table based on a magnitude of the inphase componentin case the polarities are different.

Example 36 is the pre-distortion method of one of the examples 34 or 35,wherein the at least one look-up table comprises an approximation of arotation of a polar representation of the quadrature signal to theinphase and quadrature components.

Example 37 is the pre-distortion method of example 36, wherein the atleast one look-up table comprises an approximation assumingnon-orthogonal rotations of the polar representation to obtain theinphase and quadrature components.

Example 38 is the pre-distortion method of one of the examples 36 or 37,wherein the approximation is linear.

Example 39 is the pre-distortion method of one of the examples 34 to 38,comprising determining the pre-distortion coefficients from the at leastone look-up table based on a magnitude of the quadrature component andbased on a sum of magnitudes of the quadrature component and the inphasecomponent.

Example 40 is the pre-distortion method of one of the examples 34 to 39,comprising determining the pre-distortion coefficients from the at leastone look-up table based on a magnitude of the inphase component andbased on a sum of magnitudes of the quadrature component and the inphasecomponent.

Example 41 is the pre-distortion method of one of the examples 34 to 40,comprising, in case the polarities of the inphase component and thequadrature component are equal, determining the pre-distortioncoefficients from the at least one look-up table based on a magnitude ofthe quadrature component and based on a sum of magnitudes of thequadrature component and the inphase component, and, in case thepolarities of the inphase component and the quadrature component are notequal, determining the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the inphase component and based ona sum of magnitudes of the quadrature component and the inphasecomponent.

Example 42 is the pre-distortion method of one of the examples 32 to 43,comprising determining a pre-distorted version of the inphase componentand a pre-distorted version of the quadrature component.

Example 43 is the pre-distortion method of example 42, wherein thepre-distorted version of the inphase component is an addition of theinphase component and the quadrature component weighted with thepre-distortion coefficient.

Example 44 is the pre-distortion method of one of the examples 32 to 43,wherein the predistorted version of the quadrature component is anaddition of the quadrature component and the inphase component weightedwith the pre-distortion coefficient.

Example 45 is a computer program having a program code for performingthe method of at least one of the examples 32 to 44, when the computerprogram is executed on a computer, a processor, or a programmablehardware component.

Example 46 is a machine readable storage including machine readableinstructions, when executed, to implement a method or realize anapparatus as exemplified in any pending example.

Example 47 is a machine readable medium including code, when executed,to cause a machine to perform the method of any one of examples 42 to44.

Example 48 is a communication device comprising the radio transceiver ofexample 29.

A person of skill in the art would readily recognize that steps ofvarious above-described methods may be performed by programmedcomputers. Herein, some examples are also intended to cover programstorage devices, e.g., digital data storage media, which are machine orcomputer readable and encode machine-executable or computer-executableprograms of instructions, wherein the instructions perform some or allof the acts of the above-described methods. The program storage devicesmay be, e.g., digital memories, magnetic storage media such as magneticdisks and magnetic tapes, hard drives, or optically readable digitaldata storage media. Further examples are also intended to covercomputers programmed to perform the acts of the above-described methodsor (field) programmable logic arrays ((F)PLAs) or (field) programmablegate arrays ((F)PGAs), programmed to perform the acts of theabove-described methods.

The description and drawings merely illustrate the principles of thedisclosure. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, exemplify the principles of thedisclosure and are included within its spirit and scope. Furthermore,all examples recited herein are principally intended expressly to beonly for pedagogical purposes to aid the reader in understanding theprinciples of the disclosure and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andexamples of the disclosure, as well as specific examples thereof, areintended to encompass equivalents thereof.

Functional blocks denoted as “means for . . . ” (performing a certainfunction) shall be understood as functional blocks comprising circuitrythat is configured to perform a certain function, respectively. Hence, a“means for s.th.” may as well be understood as a “means configured to orsuited for s.th.”. A means configured to perform a certain functiondoes, hence, not imply that such means necessarily is performing thefunction (at a given time instant).

Functions of various elements shown in the figures, including anyfunctional blocks labeled as “means”, “means for controlling”, “signalprocessing means”, “means for storing”, “means for inputting”, etc., maybe provided through the use of dedicated hardware, such as “acontroller”, “a processor”, “a storage or a memory”, “an input”, etc. aswell as hardware capable of executing software in association withappropriate software. Moreover, any entity described herein as “means”,may correspond to or be implemented as “one or more modules”, “one ormore devices”, “one or more units”, etc. When provided by a processor,the functions may be provided by a single dedicated processor, by asingle shared processor, or by a plurality of individual processors,some of which may be shared. Moreover, explicit use of the term“processor” or “controller” should not be construed to refer exclusivelyto hardware capable of executing software, and may implicitly include,without limitation, digital signal processor (DSP) hardware, networkprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), read only memory (ROM) for storingsoftware, random access memory (RAM), and non-volatile storage. Otherhardware, conventional and/or custom, may also be included.

It should be appreciated by those skilled in the art that any blockdiagrams herein represent conceptual views of illustrative circuitryexemplify the principles of the disclosure. Similarly, it will beappreciated that any flow charts, flow diagrams, state transitiondiagrams, pseudo code, and the like represent various processes whichmay be substantially represented in computer readable medium and soexecuted by a computer or processor, whether or not such computer orprocessor is explicitly shown.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example. While each claim may stand on its own as a separateexample, it is to be noted that—although a dependent claim may refer inthe claims to a specific combination with one or more other claims—otherexamples may also include a combination of the dependent claim with thesubject matter of each other dependent or independent claim. Suchcombinations are proposed herein unless it is stated that a specificcombination is not intended. Furthermore, it is intended to include alsofeatures of a claim to any other independent claim even if this claim isnot directly made dependent to the independent claim.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective acts of these methods.

Further, it is to be understood that the disclosure of multiple acts orfunctions disclosed in the specification or claims may not be construedas to be within the specific order. Therefore, the disclosure ofmultiple acts or functions will not limit these to a particular orderunless such acts or functions are not interchangeable for technicalreasons. Furthermore, in some examples a single act may include or maybe broken into multiple sub acts. Such sub acts may be included and partof the disclosure of this single act unless explicitly excluded.

1-25. (canceled)
 26. A pre-distortion circuit for a digital quadraturesignal, the predistortion circuit comprising: a first input for aninphase component of the quadrature signal; a second input for aquadrature component of the quadrature signal; signal processing circuitconfigured to determine the signs of the inphase component and of thequadrature component; determine pre-distortion coefficients based on theamplitude of the inphase component, the amplitude of the quadraturecomponent, and based on the signs of the inphase component and thequadrature component.
 27. The pre-distortion circuit of claim 26,comprising a generation module configured to generate the pre-distortioncoefficients.
 28. The pre-distortion circuit of claim 27, wherein thegeneration module is a storage module configured to store at least oneone-dimensional look-up table, and wherein the at least one look-uptable comprises a mapping between amplitude values and predistortioncoefficients or wherein the at least one look-up table comprises amapping between a number of activated cells in aradio-frequency-digital-to-analog converter and pre-distortioncoefficients, the radio-frequency-digital-to-analog converter beingconfigured to convert the inphase component or the quadrature component.29. The pre-distortion circuit of claim 28, wherein the signalprocessing circuit is configured to determine a pre-distortioncoefficient from the at least one look-up table based on a magnitude ofthe quadrature component in case the polarities are equal and whereinthe signal processing circuit is configured to determine apre-distortion coefficient from the at least one look-up table based ona magnitude of the inphase component in case the polarities aredifferent.
 30. The pre-distortion circuit of claim 28, wherein the atleast one lookup table comprises an approximation of a rotation of apolar representation of the quadrature signal to the inphase andquadrature components.
 31. The pre-distortion circuit of claim 30,wherein the at least one lookup table comprises an approximationassuming non-orthogonal rotations of the polar representation to obtainthe inphase and quadrature components.
 32. The pre-distortion circuit ofclaim 30, wherein the approximation is linear.
 33. The pre-distortioncircuit of claim 28, wherein the signal processing circuit is configuredto determine pre-distortion coefficients from the at least one look-uptable based on a magnitude of the quadrature component and based on asum of magnitudes of the quadrature component and the inphase component.34. The pre-distortion circuit of claim 28, wherein the signalprocessing circuit is configured to determine the pre-distortioncoefficients from the at least one look-up table based on a magnitude ofthe inphase component and based on a sum of magnitudes of the quadraturecomponent and the inphase component.
 35. The pre-distortion circuit ofclaim 28, wherein the signal processing circuit is configured to in casethe polarities of the inphase component and the quadrature component areequal determine the pre-distortion coefficients from the at least onelook-up table based on a magnitude of the quadrature component and basedon a sum of magnitudes of the quadrature component and the inphasecomponent, and in case the polarities of the inphase component and thequadrature component are not equal determine the pre-distortioncoefficients from the at least one look-up table based on a magnitude ofthe inphase component and based on a sum of magnitudes of the quadraturecomponent and the inphase component.
 36. The pre-distortion circuit ofclaim 28, wherein the signal processing circuit is configured todetermine a pre-distorted version of the inphase component and apredistorted version of the quadrature component.
 37. The pre-distortioncircuit of claim 36, wherein the pre-distorted version of the inphasecomponent is an addition of the inphase component and the quadraturecomponent weighted with the pre-distortion coefficient.
 38. Thepre-distortion circuit of claim 26, wherein the pre-distorted version ofthe quadrature component is an addition of the quadrature component andthe inphase component weighted with the pre-distortion coefficient. 39.A transmitter comprising the pre-distortion circuit of claim 26 and aradio-frequency-digital-to-analog converter configured for converting abase band signal to an analog radio frequency signal based an outputsignal of the pre-distortion circuit.
 40. The transmitter of claim 39,wherein the radio-frequency-digital-to-analog converter is a capacitivedigital-to-analog converter.
 41. A radio transceiver comprising thetransmitter of one of the claim
 39. 42. A mobile transceiver comprisingthe radio transceiver of claim
 41. 43. A base station transceivercomprising the radio transceiver of claim
 41. 44. A pre-distortionmethod for a digital quadrature signal, the predistortion methodcomprising inputting an inphase component of the quadrature signal;inputting a quadrature component of the quadrature signal; determiningsigns of the inphase component and of the quadrature component;determining pre-distortion coefficients based on the amplitude of theinphase component, the amplitude of the quadrature component, and basedon the signs of the inphase component and the quadrature component. 45.The pre-distortion method of claim 44, comprising generating thepre-distortion coefficients.
 46. The pre-distortion method of claim 45,wherein the generating comprises storing at least one one-dimensionallook-up table, and wherein the at least one look-up table comprises amapping between amplitude values and pre-distortion coefficients orwherein a at least one look-up table comprises a mapping between anumber of activated cells in a radiofrequency-digital-to-analogconverter and pre-distortion coefficients, theradio-frequency-digital-to-analog converter being configured forconverting the inphase component or the quadrature component.
 47. Thepre-distortion method of claim 46, comprising determining apre-distortion coefficient from the at least one look-up table based ona magnitude of the quadrature component in case the polarities areequal, determining a pre-distortion coefficient from the at least onelook-up table based on a magnitude of the inphase component in case thepolarities are different.
 48. The pre-distortion method of claim 46,wherein the at least one look-up table comprises an approximation of arotation of a polar representation of the quadrature signal to theinphase and quadrature components.
 49. A machine readable storageincluding machine readable instructions which, when executed, implementpre-distortion as claimed in claim
 44. 50. A communication devicecomprising the radio transceiver of claim 41.